Boiler Size Calculator
Find the right size boiler for your home. Enter your heated square footage, climate zone and insulation, choose a heat-only or combi boiler, and get your design heat load, hot-water demand, the BTU input rating you need and the nearest standard boiler size — plus output in kW, fuel use at full fire, by-zone and by-home-size tables, and a downloadable PDF.
| Climate zone | BTU / sq ft | Required output | Recommended size |
|---|---|---|---|
| Enter your home to compare | |||
| Heated area | Required output | Input needed | Recommended size |
|---|---|---|---|
| Enter your home to compare | |||
How to use the boiler size calculator
Four quick steps turn your home’s size, climate and insulation into a design heat load, the BTU input rating you need and the nearest standard boiler size — with by-zone and by-home-size tables, a chart and a downloadable report.
Describe your home
Enter the heated square footage and ceiling height, then pick your climate zone and insulation level.
Choose the boiler type
Pick heat-only, or combi and enter how many showers or taps run at once and your hot-water temperature.
Set efficiency, fuel & margin
Choose the AFUE (82%, 87% or 95%), natural gas, propane or oil, and a 0%, 10% or 20% safety margin.
Read results & save a PDF
Get the heat load, input needed, recommended size, kW output and fuel use — then download a report.
How boiler size is calculated
The calculator estimates how much heat your home loses on a cold day, adds a small margin, checks the hot-water demand for a combi, then converts the output you need into a boiler input rating. Here is the exact math it runs.
Worked example — 2,000 sq ft, 8 ft ceilings, Zone 5, average insulation, heat-only, 95% AFUE, natural gas, 10% margin
Heat loss: 2,000 × 40 × 1.00 × (8 ÷ 8) = 80,000 BTU/hr · Design load: 80,000 × 1.10 = 88,000 BTU/hr
Input needed: 88,000 ÷ 0.95 ≈ 92,632 BTU/hr · Recommended: 100,000 input × 0.95 = 95,000 BTU/hr output
Headroom: 95,000 ÷ 88,000 − 1 ≈ 8% · Output: 95,000 ÷ 3,412 ≈ 27.8 kW · Gas: 100,000 ÷ 100,000 = 1.00 therm/hr
≈ 100,000 BTU/hr input boiler — about 95,000 BTU/hr (27.8 kW) output, a good fit with 8% headroom.
Boiler size charts
Handy lookups for how climate zone and home size change the boiler you need, and how hot-water demand sizes a combi boiler.
| Climate zone | BTU / sq ft | Required output | Recommended input |
|---|---|---|---|
| Zone 1–2 (hot) | 25 | 55,000 | 60,000 |
| Zone 3 (warm) | 30 | 66,000 | 70,000 |
| Zone 4 (mixed) | 35 | 77,000 | 90,000 |
| Zone 5 (cool) | 40 | 88,000 | 100,000 |
| Zone 6 (cold) | 45 | 99,000 | 110,000 |
| Zone 7+ (very cold) | 50 | 110,000 | 120,000 |
All figures in BTU/hr. Zone 4 jumps to 90,000 because an 80,000 boiler at 95% gives only 76,000 BTU/hr output.
| Heated area | Required output | Input needed | Recommended input |
|---|---|---|---|
| 1,000 sq ft | 44,000 | 46,316 | 50,000 |
| 1,500 sq ft | 66,000 | 69,474 | 70,000 |
| 2,000 sq ft | 88,000 | 92,632 | 100,000 |
| 2,500 sq ft | 110,000 | 115,789 | 120,000 |
| 3,000 sq ft | 132,000 | 138,947 | 140,000 |
| 4,000 sq ft | 176,000 | 185,263 | 199,000 |
All figures in BTU/hr. Poor insulation adds 25%; good insulation takes off 15%.
| Showers / taps at once | Hot-water load | Sized by | Recommended input |
|---|---|---|---|
| 1 (2.0 GPM) | 70,000 | Space heating (88,000) | 100,000 |
| 2 (4.0 GPM) | 140,000 | Hot water | 150,000 |
| 3 (6.0 GPM) | 210,000 | Hot water | 225,000 |
Zone 5 groundwater is about 50°F, so heating to 120°F is a 70°F rise. Colder zones have colder groundwater and need more.
What the boiler size calculator gives you
A few details about your home give you a full sizing picture to take into contractor quotes.
Boiler sizing at a glance
Who uses a boiler size calculator
Whether you’re replacing an old boiler or switching to a combi, the same numbers help you ask the right questions.
The old boiler is failing and wants to know whether the quoted size is right — or just a copy of the old nameplate.
- Enter heated area and zone
- Compare with the old size
- Ask for a Manual J
Wants one wall-hung unit for heat and hot water and needs to know whether showers or heating set the size.
- Choose Combi
- Enter showers at once
- Check what sized it
Needs a fast ballpark at the first visit before running a full heat-loss calculation for the quote.
- Try each AFUE
- Flag oversizing
- Share the PDF
Tips for choosing the right boiler size
A right-sized boiler runs longer, steadier cycles and burns less fuel — these habits help you get there.
Boiler size calculator FAQ
Quick answers about boiler sizing, BTU ratings, AFUE, combi boilers and why bigger isn’t better.
In climate zone 5 with average insulation and 8 ft ceilings, a 2,000 sq ft home loses about 80,000 BTU/hr. With a 10% margin that’s an 88,000 BTU/hr design load, so a 95% AFUE boiler needs about 92,632 BTU/hr input — a 100,000 BTU/hr boiler.
The same home needs about a 60,000 BTU/hr boiler in zone 1–2 and about 120,000 BTU/hr in zone 7+.
Multiply the heated square footage by the BTU per sq ft for your climate zone (25 to 50), then adjust for insulation and ceiling height to get the heat loss. Add a safety margin to get the design heat load.
Divide the required output by the boiler’s AFUE to get the input needed, then choose the smallest standard boiler whose output covers your load.
Input is the fuel energy the boiler burns, and output is the heat it delivers to your home. Output equals input times AFUE.
A 100,000 BTU/hr input boiler gives 95,000 BTU/hr at 95% AFUE but only 82,000 at 82%. That’s why an 88,000 BTU/hr load needs a 110,000 BTU/hr input boiler at 82% AFUE.
A combi boiler must heat water on demand, so its size is the larger of the heat load and the hot-water load. Hot-water load is GPM × 500 × temperature rise, with each shower or tap counted at 2.0 GPM.
In zone 5, heating 50°F groundwater to 120°F is a 70°F rise. One shower needs 70,000 BTU/hr, so an 88,000 BTU/hr heat load still sets the size. Two showers at once need 140,000 BTU/hr, which calls for a 150,000 BTU/hr boiler.
Not without checking. Most older boilers were oversized, and many homes have since added insulation or new windows that cut the heat loss.
Use this calculator for an estimate, then have your contractor run an ACCA Manual J heat-loss calculation. With good insulation the 2,000 sq ft zone 5 example drops to an 80,000 BTU/hr boiler.
An oversized boiler reaches temperature quickly and shuts off, so it short-cycles. That wastes fuel, wears out parts and makes condensing boilers less efficient.
The calculator flags any size with more than 25% headroom. If that happens, look for a modulating (mod-con) boiler with a low minimum firing rate.
A 10% margin is typical and covers recovery after a setback and unusually cold nights. Use 20% only for very old, leaky homes.
For the 2,000 sq ft zone 5 example, no margin needs a 90,000 BTU/hr boiler, 10% needs 100,000 and 20% needs 110,000.
This calculator uses 25 BTU/sq ft in zones 1–2, 30 in zone 3, 35 in zone 4, 40 in zone 5, 45 in zone 6 and 50 in zone 7+, based on 8 ft ceilings and average insulation.
Poor insulation multiplies the load by 1.25, good insulation by 0.85, and taller ceilings scale it up — a 10 ft ceiling adds 25%.
Planning estimate only — not a Manual J. This calculator works in US units (BTU/hr, sq ft, °F) and uses a square-foot rule of thumb by climate zone. The correct boiler size comes from an ACCA Manual J heat-loss calculation. Don’t just match the old boiler’s nameplate, and have a licensed heating contractor size, install and vent the boiler to local code.